The controls of missing MH370 passenger jet were likely “deliberately manipulated”, Malaysian investigators have said, adding they cannot rule out “unlawful interference by a third party”.

The Malaysia Airlines Boeing 777 veered off course and exhaustive searches of the southern Indian Ocean have failed to turn up its remains.

It vanished with 239 people on board en route to Beijing from Kuala Lumpur on 8 March 2014.

Now, safety investigators have released a fresh report into the flight’s mysterious disappearance but said conclusive answers could only be gained if the wreckage is eventually found.

The only confirmed traces of the aircraft have been three wing fragments washed up along the Indian Ocean coasts.

Malaysian and international investigators have been looking into why the Boeing 777 jet veered thousands of miles off course from its scheduled route before eventually plunging into the Indian Ocean.

Experts believe someone may have deliberately switched off MH370’s transponder before diverting it over the Indian Ocean.

The last communication from the plane was from the Captain Zaharie Ahmad Shah who signed off with “Good night, Malaysian three seven zero”, as the plane left Malaysian airspace.

Kok Soo Chon, head of the MH370 safety investigation team, said as the new report was released that the history of the pilot and first officer had been examined, and experts were satisfied with their background, training and mental health.

“We are not of the opinion it could have been an event committed by the pilots,” he said, but added investigators were not ruling out any possibility since the in-air turn back was done manually and the systems in the plane were also manually turned off.

“We cannot exclude that there was an unlawful interference by a third party” such as the pilots being held hostage, Mr Kok said. But he added that no group has said it hijacked the plane and no ransom demands have been made, compounding the mystery.

He added all the passengers of the 15 countries had their backgrounds checked by their respective countries and all came back with a clean bill of health.

Mr Kok said the latest investigation showed lapses by air traffic control, including a failure to swiftly initiate an emergency response and monitor radar continuously, relying too much on information from Malaysia Airlines and not getting in touch with the military for help.

A 440-page final report by the Australian Transport Safety Bureau (ATSB) last year showed that Mr Zaharie had flown a route on his home flight simulator six weeks earlier that was “initially similar” to the one actually taken by MH370.

A forensic report by the Malaysian police previously concluded that there were no unusual activities other than game-related flight simulations.